Various types of ophthalmic apparatuses employing optical apparatuses are used. For example, optical apparatuses for observing eyes include anterior eye imaging apparatuses, fundus cameras, and confocal scanning laser ophthalmoscopes (SLOs). Especially, an optical coherence tomography apparatus (hereinafter, referred to as OCT apparatus) can acquire high-resolution tomographic images of a sample, and the apparatus has become an essential ophthalmic apparatus used in outpatient clinics specializing in retina care.
The OCT apparatus enables high-sensitivity eye measurement by irradiating a sample with low coherent light, and using an interference system of the reflected light from the sample. Further, the OCT apparatus can acquire high-resolution tomographic images by scanning the sample with the low coherent light. Since the OCT apparatus can capture high-resolution tomographic images of a retina in a fundus of a subject's eye, the OCT apparatus is widely used for an ophthalmic diagnosis for retinas, and the like.
In the ophthalmic diagnosis for retinas, a tomographic image of a retina, which is referred to as a B scan image, captured by an OCT apparatus are generally used. The B scan image is acquired by performing scanning in a depth direction (Z direction) of the retina, which is referred to as A scan, in an X direction for a plurality of times. When comparing with an image of a conventional fundus camera, a state of an inside of the retina can be observed from the B scan image, the B scan is an innovative technique for observing a lesion inside of the retina, especially, for observation of macular degeneration, a macular hole, and the like.
The B-scan image is acquired by only one scanning. However, by scanning the same point for a plurality of times and superimposing the scanned images, a sharp image with less noise can be acquired. However, human eyes move involuntary, which is referred to as involuntary eye movement during fixation. Due to the movement, a target retina involuntary moves while the same point is geometrically scanned, and accordingly, it is difficult to superimpose many images.
United States Patent Application Publication No. 2010-0157311 discusses a device for capturing a plurality of B-scan images in the Y direction to acquire a three-dimensional retinal image is developed. The acquisition of the three-dimensional retinal image contributes to the observation of the extent of a lesion and layers in a retina, especially to the observation of the ganglionic layer of optic nerve that causes glaucoma. However, in the observation, due to the movement of the eyeball, the three-dimensional retinal image may be distorted. To solve this issue, a technique referred to as tracking has been developed. In the tracking technique, a fundus image is captured while tomographic images of the retina are captured, and a feature point such as a branch point of a blood vessel in the fundus image is focused. When the feature point moves, the movement is determined to be the movement of the retina, and the scanning position of the OCT apparatus is changed.
To accurately measure the movement of the eyeball using the fundus image, it is required to perform the processing of extracting the feature point from the fundus image, searching and detecting the feature point in the images to be processed, and calculating an amount of the movement at a high speed. As the feature point in the fundus image, a macula, an optic papillary portion (hereinafter, referred to as an optic papilla), or the like is used. In an affected eye, the macula and the optic papilla are often unclear, and consequently, a blood vessel can be used as the feature point in fundus image. Japanese Patent Application Laid-Open No. 2001-70247 discusses a method for extracting a feature point of a blood vessel.
The apparatuses for capturing a fundus image include fundus cameras and scanning ophthalmoscopes. The fundus camera acquires a fundus image of the whole area at one time. The scanning ophthalmoscope acquires a fundus image by scanning the fundus with a beam. The scanning ophthalmoscope includes the SLO and line-scanning laser ophthalmoscopy (LSLO). The SLO irradiates a laser spot on a fundus, and scans the fundus with a laser beam. The LSLO irradiates and scans a fundus with a linear laser beam. The scanning ophthalmoscopes are considered to be able to provide high-quality (high-resolution, high-luminance) images while its image capturing time is longer than that of the fundus camera. Detailed configuration of the LSLO is discussed in U.S. Pat. No. 4,768,874 and United States Patent Application publication No. 2003-0231285. Generally, to measure the movement of an eyeball, a feature point is to be detected. Consequently, a scanning ophthalmoscope capable of performing high-quality continuous shooting is used.
However, in the simultaneous imaging by the OCT apparatus for tracking and by the scanning ophthalmoscope described above, the imaging light of the OCT apparatus and the imaging light of the scanning ophthalmoscope simultaneously enters the eye to be observed. Then, the light amount entering the eye increases. On the other hand, the upper limit of the incident light amount allowed for the eye observation is determined by the International Organization for Standardization (ISO), and other organizations. Accordingly, the imaging light amount of the OCT apparatus is to be adjusted to a low amount. This results in deterioration in image quality such as decrease in signal-to-noise (S/N) ratios.